In a MIMO communication system, multiple antennas are used at both the transmitter and the receiver, for example, of a base station (BS). By exploiting the spatial dimensions of the multiple antenna transceiver, concurrent transmissions of signals by multiple antennas can be realized, thus increasing the spectral efficiency and achieving higher data rates without increasing transmission bandwidth.
In general, MIMO communication systems can be broadly classified into two types: open loop systems and closed loop systems. For open loop systems, there is no feedback of channel information from the user equipment to the base station. In contrast, for closed loop systems, feedback of channel information is provided by the user equipment to the base station, and the base station adopts the transmission mode according to the feedback information.
There is a need to provide the feedback but there is also a tradeoff for doing so. For example, one advantage of having the feedback is to allow the system to adapt to the varying channel conditions in a timely manner, thus effectively improving the system performance, for example, increasing the physical layer throughput. Nevertheless, it is very challenging to feed the channel information back to base station efficiently given the amount of the channel coefficients, the small bandwidth of a feedback control channel, and the fact that the feedback control channel is shared by different users in the same system. This means when the full channel information is continuously provided to the base station, a heavy loading will be exerted upon the system, consuming system resources that could otherwise be allocated to other functions.
Attempts have been made to minimize the channel information in the feedback to reduce the heavy loading so that closed loop systems are further classified into two types: limited feedback systems and rich feedback systems. For limited feedback systems, only partial or limited channel information is sent. However, for rich feedback systems, full channel information such as full CSI (channel state information) is sent.
Another attempt to handle the tradeoff for providing feedback is, instead of providing feedback continuously, the system is switched between two modes of operation wherever appropriate: open loop mode and closed loop mode.
The U.S. Patent Application Pre-grant Publication No. US2009/0003475A1 discloses the adaption between different closed loop, open loop and hybrid techniques. The adaption is based on a selected one of: a baseline quantity of channel information in feedback (similar to rich feedback system), a reduced quantity version of the baseline quantity of channel information in feedback (similar to limited feedback system), or no channel information in feedback (similar to open loop system).
However, the existing approaches have a number of limitations and disadvantages, some of which will be illustrated below and become apparent to a person skilled in the art through comparing existing systems with some aspects of the present invention.